Engine-powered, three- or four-wheeled vehicles equipped with ultra-low pressure tires around the wheels have been practically used for off-road driving or traveling in a farm, a pasture or a desert.
In the vehicles of the type described, it has been customary that left and right driving wheels are secured to the opposite ends of a long axle shaft extending through and rotatably supported, at an intermediate portion thereof, on a tubular supporting member extending transversely between ends of left and right longitudinal support members extending from the vehicle frame along a plane of the vehicle.
According to such prior driving wheel supporting system, the axle shaft is supported at its intermediate portion so that it is subjected to bending stresses due to severe vibrations and shock applied thereto during off-road traveling. In view of the relatively short supporting pipe member, an attempt has been made to enlarge the diameter of the axle shaft as well as the thickness of the supporting pipe member to thereby increase the strength and rigidity of these components. However, such attempt has caused another problem to be solved, namely an increase in weight of the axle shaft and its related components fails to meet the desire for reduction of the overall vehicle weight, results in a high material cost, and causes a substantial power loss.